Invertebrates belonging to the largest phylum of animals, Arthropoda, with an exoskeleton, segmented bodies, and jointed appendages, including many subphyla and classes, such as insects, crustaceans, horseshoe crabs, sea spiders, centipedes, millipedes, and the extinct trilobites.

National Wildlife Health Center studies the West Nile Virus to learn the current geographic extent, to understand how the virus moves between birds, mosquitoes, and humans, and to predict future movements of the virus.